Seismic array with spaced sources having variable pressure

ABSTRACT

An over/under seismic source system includes a first umbilical to a first gun array at a first depth and a second umbilical at a different air pressure to a second gun array at a second, lower depth. The air pressure to the second, lower gun array is tuned so that the periods of the gun bubbles from the higher and lower gun arrays match in order to improve wavefield separation in subsequent data processing.

This application claims the benefit of U.S. Patent Application Ser. No.60/826,616 filed Sep. 22, 2006.

FIELD OF THE INVENTION

The present invention relates generally to the field of seismicsurveying and, more particularly, to a set of vertically spaced apartseismic sources for use in seismic surveying, including use with a towedarray, as well as ocean bottom cables and node surveys, wherein thespaced apart sources operate at different pressures and volumes.

BACKGROUND OF THE INVENTION

In conventional towed-streamer marine acquisition, a plurality ofseismic streamers are towed behind a vessel at a desired depth. Each ofthe plurality of streamers comprises many sensors of one or more typesto acquire seismic data, as well as a number of external devices such asbirds to maintain the streamers at the desired depth and one or moresources to generate the seismic signals.

It is well known that shallow sources and cables increase thehigh-frequency content of the seismic data that is needed forresolution. However, this arrangement attenuates the low frequenciesneeded for stratigraphic and structural inversion. Towing the seismicstreamers at a shallow depth also makes the data more susceptible toenvironmental noise, typically from the towing vessel and noisegenerated at the water surface from wave action, wind, rain, and otherenvironmental sources.

Conversely, deep sources and deep cables enhance low frequencies,attenuate high frequencies, and the data has a highersignal-to-ambient-noise ratio due to a reduced susceptibility toenvironmental noise. Low frequency content of the seismic signal isimportant for penetration of deep geologic structures. A conventionaltowed-streamer survey design, therefore, attempts to balance theseconflicting aspects to arrive at a tow depth for sources and receivercables that optimizes bandwidth and signal-to-noise ratio for a targetdepth or two-way travel time, often at the expense of other shallower ordeeper objectives.

An over/under, towed-streamer configuration acquires seismic data withcables towed in pairs at two different cable depths, with one cableabove the other. In a similar manner, it is possible to acquire datawith paired sources at two different source depths. However, in thissituation there are two opposing effects that influence the frequencycontent of the signals generated by the sources. Sources operating atlower depths exhibit a higher natural frequency of bubble reverberationthan sources at shallower depths, while the effect of the ghostreflection from the water-air interface emphasizes the lowerfrequencies. These effects thereby create a frequency mismatch in thesignals generated by the over/under sources.

The over/under acquisition concept using wave field separation has beenknown and understood since the mid-1980s. The success of the wave fieldseparation method depends on accurately maintaining the over/understreamers in the same vertical plane. In original systems, thisrequirement was too difficult to fulfill and consequently this very goodgeophysical idea was shelved for some time. Recent commercialapplications of the over/under technique have been made possible by thedevelopment of steerable streamers. The control systems associated withthese cables are capable of keeping them in horizontal and verticalalignment, one above the other, to within the small tolerance requiredfor the method to work correctly.

In one known system, data from an over/under streamer and sourcesconfiguration are combined at the processing stage into a single datasetwhere both the lower source and streamer ghosts are removed. Thus, theresulting dataset has the high-frequency characteristics of conventionaldata, recorded at a shallow towing depth, and the low-frequencycharacteristics of conventional data, recorded at a deeper towing depth.

There are a number of benefits to over/under data compared toconventional data. First, significantly broader signal bandwidth withlow-frequency content gives deeper penetration down into geologicstructures underlying the ocean bottom, and therefore, improved imagingbeneath basalt, salt and other highly absorptive overburdens. Moreover,the bandwidth extension to lower frequencies makes seismic inversionless dependent upon model-based methods. Second, if the over/under cablepair defines a vertical separation equal to the shallow tow depth of aconventional high-resolution configuration (typically less than sixmeters), and the closely spaced over/under cable pair were towed atdepth (typically greater than fifteen meters), then the combinedover/under dataset would have the high-frequency content given by thevertical cable separation and the low-frequency content delivered by thedeep tow depth.

The over/under arrangement includes a number of other advantages. Forexample, a simpler signal wavelet with the bandwidth extension to higherfrequencies gives enhanced resolving power and allows for a moredetailed stratigraphic interpretation. The deeper towed-cable pairsprovide a higher signal-to-ambient-noise ratio. In addition, the deepertowed-cable pairs enable an extended weather window. Finally, theover/under data may in future offer ocean-bottom-cable typemultiple-attenuation schemes to towed streamer data and enable theremoval of sea-surface effects from three-dimensional data, hence,improving four-dimensional repeatability.

The principles of the over/under source configuration follows those ofthe over/under cable. Two source arrays are deployed at differentdepths. Once again, the wave field separation method requires constantdepths with constant vertical separation and no lateral separationbetween the geometrical centers of the arrays.

Unfortunately, in an over/under source configuration, the lower sourceis subjected to a higher hydrostatic pressure, resulting in a mismatchin the bubble period and the peak-to-bubble ratio as well of the higher(over) source and the lower (under) source.

Thus, there remains a need for an over/under system in which the signalprofiles of the respective arrays can be tuned to eliminate this factorof interference between the signals. In this way, the upper and lowerarrays can have identical wave shapes, resulting in a simpler operatorfor the wave field separation and therefore resulting in clearer imagesof the geological data.

SUMMARY OF THE INVENTION

The present invention addresses these and other needs in the art byproviding at least two seismic sources, spaced vertically apart,operating at different air pressures. In a preferred embodiment, a firstumbilical feeds a first gun array at a first depth and a secondumbilical at a different air pressure feeds a second gun array at asecond, lower depth. The air pressure to the second, lower gun array istuned so that the periods of the gun bubbles from the higher and lowerguns match. In so doing, the unghosted signature of the lower source canbe seen as a time delayed version of that of the upper source with aproportional amplitude due to the different pressure. Thus, the wavefield separation operator becomes more suitable for subsequent dataprocessing.

In another preferred embodiment, pairs of gun arrays, fed fromindependent umbilicals, are deployed in one array. The guns may bepositioned vertically over one another, in which case the guns are firedsimultaneously or very nearly simultaneously so that the downgoingsignals from the respective sources are synchronized. Alternatively, thelower guns may be staggered in a vertically and horizontally displacedarrangement, in which case the lower guns are fired at a delayed time,depending on the speed of traverse of the arrays.

These and other features and advantages of this invention will beapparent to those of skill in the art from a review of the followingdetailed description along with the accompanying drawing figure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a marine seismic system includingover/under signal sources with independent umbilicals which operate atdifferent pressures.

FIG. 2 is a schematic diagram of a presently preferred over/under signalsource system in which the upper and lower sources are oriented along avertical orientation for simultaneous firing of source arrays.

FIG. 3 is a detail view of the system of FIG. 2.

FIG. 4 is a schematic diagram of a presently preferred over/under signalsource system in which the upper and lower sources are staggered forsequential firing of source arrays.

FIG. 5 is a schematic diagram illustrating seismic signal paths from asingle source to an over/under seismic streamer.

FIG. 6 is a schematic diagram illustrating seismic signal paths from anover/under source configuration to a streamer.

FIG. 7 is a time plot depicting the mismatch in the bubble period fromover and under sources operating at the same pressure and volume.

FIG. 8 is a frequency response plot showing unmatched amplitude spectrafrom the over and under sources operating and the same pressure andvolume.

FIG. 9 is a time plot of an unghosted signature comparison of tunedsources in accordance with the present invention prior to scaling.

FIG. 10 is a time plot of an unghosted signature comparison of tunedsources in accordance with the present invention after scaling.

FIG. 11 is a plot of unghosted amplitude spectra of the presentinvention prior to scaling.

FIG. 12 is a plot of unghosted amplitude spectra of the presentinvention after scaling.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The system depicted in FIG. 1 is a data acquisition and control system10 designed for marine seismic operations, including an over/undersignal arrangement in accordance with the present invention, asdescribed in greater detail below. The system includes a shipboardcontroller 14 aboard a vessel 16 and in-water remote units 12.

The in-water remote units 12 include air gun arrays 18, which may be ofmany well known types. Accompanying each gun array 18 is a set of gunacoustic units 20 and associated tow fish acoustic units 22 deployedbelow. The system also includes a tail buoy 24 and an associated towfish acoustic unit 26 at the end of each streamer 30, for example.

The system may also include a tow buoy 28 and either a gun acousticsunit or a surface mount unit. The gun acoustics unit provides preciselocation of the seismic energy source relative to a fixed referencepoint, while the surface mount acoustic unit is used in locations wherethe use of streamer-mounted remote units would be impractical.

Spaced along each streamer is a plurality of depth control devices 32,commonly referred to as birds. A depth control device 32 may alsoinclude a float tube 34 attached to it. Also spaced along each streameris a set of modules 36. The modules provide coupling for high data ratetransmission of seismic data and telemetry accumulated by theappropriate external devices. Finally, spread out along each section ofstreamer 30 are disposed a plurality of hydrophones 38 to receiveseismic signals, perhaps thousands of such hydrophones 38. It is to beunderstood that the present invention is equally applicable to othertypes of sensors and deployment configurations.

To this point, the system shown and described is similar to that ofChien, U.S. Pat. No. 6,011,753, incorporated herein by reference.However, in the present invention, an air gun array 18′, which may alsobe of many well known types, is deployed below the air gun array 18 in avertically spaced apart relation thereto. It is to be understood that acorresponding gun array is included along the port side streamer but isnot illustrated in FIG. 1 for purposes of simplicity of illustration.Accompanying each gun array 18′ is a set of gun acoustic units 20′ andassociated tow fish acoustic units 22′ deployed below.

The gun array 18 is provided with an umbilical 40, which provides amongother things a supply of compressed air at a first air pressure. The gunarray 18′ is also provided with an umbilical 40′ to supply the gun arraywith compressed air at a different pressure than that provided to theshallower gun array 18. In this way, the acoustic signature provided bythe run array 18′ can be tuned to match the acoustic signature of thegun array 18, despite the greater hydrostatic pressure at the gun array18′. The umbilicals 40 and 40′ also include command and control signalconductors to control the timing of the firing of the guns in therespective arrays.

The system also includes an under streamer 30′, including the sameexternal devices as the over streamer 30, but operating at a lowerdepth.

It is also to be understood that other orientations and structures maybe used to provide the lower seismic signal source with a differentpressure than that of the higher seismic signal source. For example, asingle umbilical may be used, providing the different pressure air tothe lower source, then a feed line run to the higher source, with atunable pressure regulator included in the feed line.

Such an arrangement is depicted in FIG. 2 and FIG. 3. The source arrayof FIG. 2 comprises an over arrays 18 and an under array 18′. The arraysare towed behind the vessel 16 by a tow cable 17, which includes thestress members to secure the arrays the vessel, as well as power,communications conductors, and air hoses. Referring more specifically toFIG. 3, the over array 18 and the under array 18′ are coupled to a towedcarriage 50 which is pulled by the two cable 17. In this instance, afirst umbilical 40, supplied at a first air pressure, feeds compressedair to the over array 18, and a second umbilical 40′ feed compressed airto the under array 18′ at a second pressure. It is to be understood thatone umbilical could be provided to both arrays, with an air pressureregulator between them to vary the air pressure from the under array tothe over array. A series of lanyards 52 secures the over array 18 ofguns to the carriage 50 and a series of lanyards 52′ secures the underarray 18′ of guns to the carriage 50.

Finally, FIG. 4 illustrates a presently preferred embodiment, whereinthe over array 18 is staggered horizontally from the under array 18′. Inthis arrangement, the gun arrays are triggered sequentially so that thesource locations match geographically.

Over-Under Towed Streamers

Now that the structure of the present invention has been described indetail, the advantages of the present invention will be more fullyunderstood from a review of the following illustrations. FIG. 5illustrates the seismic signal ray paths from a source (gun unit) 20toward an over sensor 38 and an under sensor 38′, as previouslydescribed. The ray paths are reflected by the air/water interface at asea surface 60 and a bottom reflecting surface 62, as shown. The signalsthat are received by the sensors 38 and 38′ are made up of downgoingwaves 64 and 64′ and upgoing waves 66 and 66′.

Considering the ray paths displayed in FIG. 5, seismic wavefieldsR^(Over) and R^(Under) can be written as the sum of an up-goingwavefield and a down-going wavefield:

R ^(Over)=(R ^(Over))_(up)+(R ^(Over))_(down)  (1)

R ^(Under)=(R ^(Under))_(up)+(R ^(Under))_(down)  (2)

The wavefield separation technique consists in finding the unghostedpart of the wavefield, say (R^(Over))_(up). The upcoming wavefield atthe over streamer 38 is received later than at under streamer because ithas traveled through an extra thickness of water, Δz. Similarly, thedowngoing wavefield at under streamer 38′ is received later than at overstreamer. Provided that the over streamer and the under streamer can bekept vertically paired, wavefields at the different depths can berelated using a wave extrapolator W (angle-dependent time-shiftingfilters). Thus, the wavefield components at different depths can beexpressed as

(R ^(Over))_(up) =W×(R ^(Under))_(up)  (3)

(R ^(Under))_(down) =W×(R ^(Over))_(down),  (4)

W=exp(jk _(z) Δz)  (5)

in which W is the wave extrapolator that depth advances up-going wavesor depth delays down-going waves over a thickness of |Δz|, k_(z) denotesthe spatial frequency over the depth axis and j is the complex imaginaryunit.

Now, inserting equations (3) to (5) into equations (1) and (2) andnoting that

W ⁻¹ =W*  (6)

where the superscript * denotes the complex conjugate, the separatedwavefields are given by

$\begin{matrix}{( R^{Over} )_{up} = \frac{{WR}^{Over} - R^{Under}}{W - W^{*}}} & (7) \\{( R^{Over} )_{down} = {- \frac{{W^{*}R^{Over}} - R^{Under}}{W - W^{*}}}} & (8)\end{matrix}$

The numerator involves subtracting the deeper wavefield from thedepth-shifted shallow wavefield; this is equivalent to a ghost whichnotch corresponds to the separation between the two streamers.Furthermore, the denominator represents an optional deconvolution ofthis new ghost.

Over-Under Sources

The principles of the over/under source configuration, as shownschematically in FIG. 6, follow those of the over/under cable. Twosource arrays are deployed at different depths; again, the wave fieldseparation method requires constant depths with constant verticalseparation and no lateral separation between the geometrical centers ofthe arrays.

Considering that the over-under streamer combination has been achievedfor each source, FIG. 6 displays the ray paths for an over-under sourceconfiguration. for an over-under towed source configuration. The ghostedinput signals are shown in FIG. 6 as ray 68 and ray 68′, and theunghosted input signals are shown as rays 70 and 70′.

For any seismic response R and any input signal S, the Green's functionG is defined as follows:

R=G*S  (9)

Introducing equation (9) into equation (7), seismic response induced bythe over and the under source can be written as follows:

$\begin{matrix}{\lbrack ( R^{OverStreamer} )_{up} \rbrack^{OverSource} = {\frac{{WG}^{OverStreamer} - G^{UnderStreamer}}{W - W^{*}}S^{Over}}} & (10) \\{\lbrack ( R^{OverStreamer} )_{up} \rbrack^{UnderSource} = {\frac{{WG}^{OverStreamer} - G^{UnderStreamer}}{W - W^{*}}S^{Under}}} & (11)\end{matrix}$

Where G^(Over Streamer) and G^(Under Streamer) are the Green's functionat the over and under streamer level respectively.

Seismic inputs S^(Over) and S^(Under) are sums of a unghosted part(downgoing wavefield) and a ghost (upgoing wavefield).

S ^(Over)=(S ^(Over))_(unghosted)+(S ^(Over))_(ghost)  (12)

S ^(Under)=(S ^(Under))_(unghosted)+(S ^(Under))_(ghost)  (13)

Provided that constant towing depths with constant vertical separationand no lateral separation between the geometrical centers of the arrayscan be achieved, the different components of S^(Over) and S^(Under) arerelated so that

(S ^(Over))_(unghosted) =Y×exp(jk _(z) Δzs)×(S^(Under))_(unghosted),  (14)

(S ^(Under))_(ghost) =Y×exp(jk _(z) Δzs)×(S ^(Over))_(ghost)  (15)

where Y is the source extrapolator and Δzs is the sources' verticalseparation.

Then introducing equations (14) to (15) into equations (12) and (13),the following relationship is established:

$\begin{matrix}{{{Y \times \lbrack ( R^{OverStreamer} )_{up} \rbrack^{OverSource}} - \lbrack ( R^{OverStreamer} )_{up} \rbrack^{UnderSource}} = {\quad{{\lbrack \frac{{WG}^{OverStreamer} - G^{UnderStreamer}}{W - W^{*}} \rbrack \lbrack {Y - Y^{*}} \rbrack}( S^{Over} )_{unghosted}}}} & (16)\end{matrix}$

The term [Y−Y*] is equivalent to a ghost which notch corresponds to Δzs.Thus, as long as the unghosted far-field signature of the sources areknown, equation (16) provides a means to combine over and under sourcedatasets so that over and under source ghosts are removed.

In previous systems, over and under sources have the same volume andpressure; because the under source is subjected to a higher hydrostaticpressure, its spectrum is quite different from the over source, as shownin FIGS. 7 and 8, leading to an intricate expression for the sourceextrapolator Y. In FIG. 7, trace 72 illustrates the time response of theunder source with a 5085 in³ shot at a depth of 20 meters at 2000 psi.The trace 74 is for the over source as 12 meters, at the same volume andpressure. In FIG. 8, trace 76 shows the frequency response for the undersource and trace 78 shows the frequency response for the over source,with the data as described in respect of FIG. 7.

In contrast, the present invention provides for matching the wave shapesof the over and under sources, resulting in a simpler operator for thewave field separation and therefore in clearer images of the geologicaldata. This can be achieved by tuning the under sources so that theperiods of the gun bubbles from the higher and lower guns match, asshown in FIGS. 9, 10, 11, and 12. In FIG. 9, trace 80 illustrates thetime response of the under source with a 6350 in³ shot at a depth of 15meters at 3000 psi. while trace 82 shows the time response of the oversource with a 4740 in³ shot at a depth of 10 meters at 2000 psi. In FIG.11, trace 84 illustrates the frequency response from the under sourceand trace 86 illustrates the frequency response from the over source,with the conditions as described above in respect of FIG. 9.

In so doing, not only volumes but firing pressures may be modified, andthen the unghosted signature of the under source can be seen as a timedelayed version of that of the over source with a proportional amplitudeA due to the different pressure and volume; in other words, the sourceextrapolator can be written as

Y=A·exp(jk _(z) Δzs).  (17)

Such an expression for Y greatly simplifies the implementation ofequation (16).

By now, it should be evident that the over/under source configuration ofthe present invention find application in a variety of sensorconfigurations, including towed cable sensor arrays. However, thesensors may alternatively be mounted in autonomous seafloor nodes orthey may be deployed in a well borehole.

The principles, preferred embodiment, and mode of operation of thepresent invention have been described in the foregoing specification.This invention is not to be construed as limited to the particular formsdisclosed, since these are regarded as illustrative rather thanrestrictive. Moreover, variations and changes may be made by thoseskilled in the art without departing from the spirit of the invention.

1. An over/under seismic source system comprising: a. a first gun arrayat a first depth operating at a first air pressure for generating afirst seismic signal; and b. a second gun array at a second, lower depthoperating at a second, different air pressure for generating a secondseismic signal.
 2. The source system of claim 1, further comprisingmeans for actuating the first and second gun arrays so that the firstand second seismic signals are synchronized.
 3. The source system ofclaim 1, further comprising means for actuating the first and second gunarrays sequentially.
 4. The source system of claim 1, wherein the firstand second gun arrays are vertically arranged one over the other.
 5. Thesource system of claim 1, wherein the first and second gun arrays arehorizontally staggered from one another.
 6. The source system of claim1, further comprising a towing apparatus to tow the source system behinda vessel.
 7. The source system of claim 1, further comprising a firstumbilical mechanically coupled to the first gun array and a secondumbilical mechanically coupled to the second gun array.
 8. A seismicsystem comprising: a. a plurality of seismic sensors; b. a plurality ofgun arrays, at least two of the plurality of gun arrays verticallyspaced apart; and c. a separate umbilical to each of the least two ofthe plurality of gun arrays, the separate umbilicals carrying air atdifferent pressures.
 9. The seismic system of claim 8, wherein thesensors are mounted in towed arrays.
 10. The seismic system of claim 8,wherein the sensors are mounted in ocean bottom cables.
 11. The seismicsystem of claim 8, wherein the sensors are mounted in autonomousseafloor nodes.
 12. The seismic system of claim 8, wherein the sensorsare deployed in a well borehole.